The present invention relates to a novel organoborosilicon polymer or, more particularly, to a novel organoborosilicon polymer of which the main chain is composed of the elements of silicon, carbon, boron and nitrogen and also relates to a method for the preparation of such a novel polymer.
In recent years, a variety of organosilicon polymers and copolymers have been developed and preparation of ceramic materials mainly composed of silicon carbide in the form of, for example, fibers is proposed starting with such an organosilicon polymer by the thermal decomposition polymerization or pyrolysis thereof. Several methods have been disclosed including, for example, a method in which a cyclic or linear organopolysilane is heated at a temperature of 300.degree. to 2000.degree. C. to effect pyrolysis and polycondensation thereof giving a high-molecular polycarbosilane (see Japanese Patent Publication No. 57-26527), a method in which such a high-molecular polycarbosilane is subjected to spinning and infusibilization to give fibers of silicon carbide (see Japanese Patent Publications Nos. 57-53892 and 57-56566) and a method in which such a polycarbosilane is used as a binder of ceramic materials (see Japanese Patent Kokai No. 52-40509). Further, attempts have been made to introduce another metallic or metalloid element into the polycarbosilane compound including a method to introduce a structure of borosiloxane (see Japanese Patent Kokai No. 54-61299), a method to introduce a structure of titanium alkoxide (see Japanese Patent Kokai No. 56-74126) and a method to introduce a structure of zirconoalkoxide (see Japanese Patent Kokai No. 56-92923) and possibilities are proposed to utilize such a modified compound for the preparation of inorganic fibers or as a binder of ceramic materials in just the same manner as the unmodified polycarbosilane compounds.
From the standpoint of industrial production, however, the known methods for the preparation of a polycarbosilane compound involve several disadvantages and difficulties. While a polycarbosilane is produced by the treatment of a polysilane in a pressurizable vessel such as an autoclave under a high pressure of 80 to 130 atmospheres and at a high temperature of 400.degree. to 480.degree. C. in a batch-wise reaction or by the heat treatment for 20 to 50 hours at a high temperature of 600.degree. to 800.degree. C. in a circulation-type reactor, the yield of the desired polycarbosilane is usually low and the reaction vessel or reactor must withstand the above mentioned reaction conditions prolongedly. With an object to present a solution for these problems, an improved method is proposed in Japanese Patent Kokai No. 57-83529 in which a polysilane is first converted into a low-molecular polycarbosilane which is then polymerized into a high-molecular polycarbosilane compound but such a method can afford no true solution of the problems because of the decreased productivity due to the complicated reaction process.
A problem in the above described method of introducing another metallic or metalloid element into a polycarbosilane is the relatively inferior properties, in particular, at high temperatures of the inorganic fibers and the ceramic materials prepared therefrom or therewith. The reason therefor is that the heteroatoms such as boron, titanium and zirconium introduced into the polycarbosilane formed of the linkage of --Si--C-- in the main chain are always connected to the silicon atom through an intervening oxygen atom forming the linkages of --Si--O--B--, --Si--O--Ti-- or --Si--O--Zr-- so that the resultant inorganic fibers and ceramic materials necessarily contain such oxide linkages.